Cable is a widely used transmission medium in the communications industry. Typically, a communications cable includes a core comprising a transmission medium and a sheath system. The transmission medium may be an insulated metallic conductor or an optical fiber. The sheath system generally includes a plastic jacket. A commonly used plastic material for the jacket in outside plant communications cables is polyethylene.
For reasons which should be apparent, cables are marked. For example, the markings may be used to provide information such as the name of the manufacturer, type of cable and footage length. Of course, these markings should be permanent and not be removeable by normal contact with expected objects or by handling or by exposure to the elements.
Inks have been used for some time to mark cable jackets. The ink may be applied by a contact wheel printer. A currently used cable ink printer includes a computer controlled system that directs jet streams of ink in dot matrix, alpha-numeric codes onto the outer plastic cable jacket. The length accuracy of the markings can be adjusted to .+-.0.1% by changing electronically an input signal from a counter wheel. From a manufacturing point of view, this system appears to be a significant improvement over a previously used contact ink marker.
Marking with inks on polyethylene has always been a problem for the industry because of the wax-like surface of polyethylene and the lack of polar groups in its molecular structure which would facilitate adhesion thereto of a marking material. Unfortunately it has been found that ink jet marks are removed easily during routine cable handling. This, of course, leads to customer complaints and may result in lost business for a cable manufacturer. Long sought after has been a cost-effective, indented, permanent marking for cable jackets.
Currently, processes other than those which involve the application of ink to a cable jacket are in use in copper and in optical fiber cable manufacture, but even these suffer some disadvantages. For example, in order to mark permanently cable jackets, hot stamp apparatus has been used. Such apparatus is mechanically driven and produces high quality marks, but it is relatively expensive to operate. The capital costa and the running costs due to foil consumption cause the system to be significantly more expensive to operate than other systems. Because the unit is not computer-controlled, cable codes must be changed using print type. This is a time-consuming process. Another disadvantage of the hot stamp marker is the difficulty of establishing a correct footage measurement of a desired accuracy. Also, the marking of cable jackets with embossed letters is known in the art.
In another approach, numerical and/or alphabetical recesses or indentations have been formed by lasers in plastic material comprising cable jackets. One example of a cable marking method and apparatus using lasers is disclosed in U.S. Pat. No. 4,370,542 which issued on Jan. 25, 1983.
It has been recognized that laser units hold promise as future marking systems. The laser has no consumables and is very reliable because of an all electronic construction. Inasmuch as there are no mechanical wear points, maintenance requirements for a laser cable marking system are negligible. Loading cable product codes may be accomplished by using a keyboard, or to reduce human error, by using a bar code reader. In one laser marking system, as many as seventy-five characters can be stored in the laser's buffer memory. As the laser is activated by a line counter, a five by seven dot matrix marking is burned into the cable jacket. The marking is synchronized to the manufacturing line speed through an optical encoder.
Also, markings which contrast with the color of the cable jacket are known in the prior art. For example, in U.S. Pat. No. 4,808,966there is disclosed a cable jacket marking system which comprises two layers. A first layer includes a colorant material which is contrastingly different from the color of an overlaying second layer of plastic material. Portions of the overlaying second layer of plastic material are removed to form a desired marking which generally is repetitive along the length of the cable. The portions of the plastic material of the second layer are removed to a depth which extends to the first layer thereby exposing the first layer and providing a color contrast for the letters and/or numbers of a marking pattern in the second layer. The removal of portions of the second layer to form an alpha-numeric marking, for example, may be accomplished with a laser. Although this last-mentioned arrangement is effective to render the marking legend more discernible, it involves the use and application of another layer of plastic material.
Similarly, in the art of printed wiring boards, identifying markings are provided on a board by a process which involves the coating of a board with a material different in color from that of the board. Then, portions of the coating are burned away to have a discernible, predetermined marking remain.
What is needed and what seemingly is not provided in the art is a cable having a marking which is substantially permanent, which includes a system that does not require additional layers of material and which can be applied in existing manufacturing processes. Apparatus required for making the sought-after product should be easily intergrated into apparatus of existing manufacturing lines.